There are a variety of ways to express honesty in conversation, outside of the popular phrase "tell you the truth". To show sincerity, a speaker might say "to be honest", "frankly speaking", or "in all transparency". Alternatively, one might use "to be real with you", "between you and me", or "truth be told". Depending on the context of the conversation, some may choose to offer a "heart-to-heart", a "candid discussion", or a "straightforward opinion". Regardless of phrasing, it is important to communicate honestly and with integrity in all interactions.